Assessment of healing in chronic wounds is gaining importance as new and expensive wound treatments are brought to market. A wide variety of chronic wound treatments such as topical growth factors, bioengineered skin equivalents, negative pressure wound therapy, and hyperbaric oxygen therapy are commercially available and clinical studies of these products have shown some evidence of improved healing compared to standard of care. However, the effectiveness of each treatment is not the same in all patients, so rapid and accurate evaluation of healing progress in each individual is critical so that unsuccessful treatments can be discontinued and alternate treatments initiated as soon as possible. Reliable methods of evaluating wound healing would benefit both wound clinics by reducing the duration and cost of treatment, and the wound research community in the evaluation of clinical trials.
The main limitation of traditional wound evaluations is that they can give information mostly from the surface of the wound. Such surface characteristics of a wound do not take into account the health of the wound environment beneath the surface in the whole wound bed, and provide inadequate information regarding the wound healing status of a wound. Therefore, misdiagnosis may occur or treatment may not be altered as early as possible, with direct implications on the quality and cost of care for chronic wounds. For example, image analysis of wound pictures for color or texture pertains strictly to surface information and optical methods such as Diffuse Reflectance Spectroscopy (DRS) or Optical Coherence Tomography (OCT) can penetrate to only approximately 1 millimeter. Non-invasive analysis of the full depth of the chronic wound bed could provide the clinician with a more complete picture of wound health, allowing better prediction of wound closure and wound recurrence than can be achieved by surface measurements alone.
Several human studies have been conducted in an attempt to non-invasively characterize tissue beneath the surface of chronic wounds. High frequency ultrasound (HFUS) at frequencies in the range of 20 MHz permits high resolution (microscopic-level) imaging of skin at depths of up to 2 cm. A preliminary study showed that HFUS could be used to image structural features beneath the surface of human chronic wounds and qualitative comparisons were made with healthy skin. HFUS was used to measure skin thickness in several types of human chronic wounds (diabetic, venous, pressure, and ubiquitous ulcers), and a later study by Dyson et al. described in “Wound healing assessment using 20 MHz ultrasound and photography,” Skin Research and Technology, 2003, Vol. 9, pages 116-121, demonstrated the use of HFUS to calculate the width and depth of small acute wounds that were created experimentally in human subjects. However, it is unclear how this method would translate to chronic wounds that are very different in shape, size, and also have more ambiguous boundaries than acute wounds.
Optical Coherence Tomography (OCT) is a non-invasive imaging modality that uses low coherence interferometry to create high resolution cross-sectional images of structural features in human skin at depths of up to 1.2 mm. This method has not yet been used to image human wounds, but structures visible in OCT images of experimentally-created animal wounds have been qualitatively correlated to histological micrographs of the same wounds, and an automated imaging algorithm was developed to calculate the size of these acute animal wounds. In another animal study, polarization-sensitive OCT was used to monitor temporal changes in collagen birefringence during healing, and measurements of birefringence were shown to be greater in chemically accelerated wound healing as compared to chemically impaired healing. As with HFUS, the clinical utility of OCT as a wound monitoring methodology is uncertain due to the size and complexity of human chronic wounds.
Laser Doppler Flowmetry (LDF) and its modified methodology of Laser Doppler Imaging (LDI) are optical methods that rely on frequency shifts of an incident light beam (typically a laser in the near infrared wavelength range) to determine a quantitative index that is related to the average velocity and number of red blood cells within a tissue volume. Some researchers have used LDF and LDI to quantify relative values of cutaneous blood flow in human chronic wounds. These studies identified regions of increased blood flow within chronic wounds that may correlate to granulation tissue; however, changes in blood flow were not monitored over time. The clinical utility of LDF and LDI for serial assessment of chronic wounds is limited due to low penetration depths (˜1-2 mm) and issues with light reflection caused by curvature of the feet and presence of moisture on the surface of the wound.
Diffuse Reflectance (or Remittance) Spectroscopy (DRS) is an optical method that uses light at visible and near infrared wavelengths (400 to 1500 nm) to measure hemoglobin concentration and oxygenation of blood in superficial capillaries, to depths of approximately 1 mm. DRS spectra from chronic leg ulcers (both venous and arterial) have been empirically correlated to qualitative wound scores assessed by physicians, and changes in oxygen saturation were measured over the course of healing using DRS in diabetic foot ulcers. However, changes of the surface appearance due to bleeding and other reasons will significantly affect the capability of DRS to provide on its own information about the wound status and oxygenation.
Generally speaking, the determination of wound surface area is highly inaccurate and subjective. (See Robson, M. C., et al., “Wound Healing Trajectories as Predictors of Effectiveness of Therapeutic Agents,” in Archives of Surgery. 2000, Am Med. Assoc. p. 773-777). Wound edges may be hard to determine because of complex wound geometry. Width and depth measurements may vary from between observers during the same clinic session and are highly inaccurate between visits. Surface area does not take into account changes in wound volume. Ultrasound measurements and image analysis of digital photos provide more accurate information but are difficult to use in a busy clinical setting.
In previous publications of the present inventors, it has been reported that near infrared optical measurements correlated with wound area reduction and were able to distinguish between a diabetic wound and a non-diabetic wound in a rat model. Weingarten, M. S., et al., “Measurement of optical properties to quantify healing of chronic diabetic wounds,” Wound Repair and Regeneration, 2006, Vol. 14(3): pp. 364-370. As will be explained herein, the inventors have expanded upon this research by combining Near Infrared (NIR) with Diffuse Reflectance spectroscopy (DRS) and reporting whether the near infrared absorption coefficient correlates with histological changes in the wounds and whether the DRS scattering function correlates with collagen concentration in the healing tissue.
Moreover, it is established that wounds, burns and lesions need oxygen to heal and that ischemic conditions represent impaired healing environments. Therefore, by measuring oxygenated hemoglobin, deoxygenated hemoglobin, and oxygen saturation, the inventors suggest that it is possible to predict wound healing. Current methods in clinical wound care practice rely on estimates of the surface area by measuring length and width of the lesion. These methods are highly subjective and more importantly cannot assess the probability of wound healing in impaired environments, such as in chronic wounds due to diabetes, venous ulcers, pressure ulcers, ubiquitous ulcers, and others. Invasive monitoring based on biopsies could provide information about the physiology and biochemistry of healing but is invasive and impractical, while monitoring based on wound fluid is controversial due to debates over appropriate correlation of wound fluid composition to wound tissue.
At present, various optical methods have been proposed and can be used for determining parameters representing skin injury or for monitoring the healing processes. Most optical methods are non-invasive and relatively inexpensive and as such offer major advantages compared to invasive methods. Different modifications of diffuse reflectance spectroscopy (DRS) have become the most common methodology in monitoring wounds, burns and lesions. DRS has been used extensively for evaluating skin changes at superficial depths up to 1 mm because with a typical broad range wavelengths source of incident light (400-1500 nm) the strong absorption exhibited by the tissue inhibits optical probing of deeper layers. Using specialized algorithms to fit DRS re-emission spectra to phantoms and model systems, many investigators obtained important information about the depth of burn injuries, sun damage, topical drug delivery, and water content of the skin.
In wound characterizations, the absence of significant depth penetration makes DRS data difficult to interpret. For example, DRS data from a significant number of wounds had to be collected in order to develop an empirical algorithm that could mimic a clinical wound assessment score which averages clinical observations. In order to probe deeper tissue depths with optical non-invasive methods, a different approach than DRS is desired. Such an approach is described herein.